1. Field of the Invention
The present invention pertains to concentric drill rod assemblies for percussion drills having an inner rod member which transmits only percussion blows and an outer tubular sheath which transmits torque from the drill to the bit or a bit adapter member.
2. Background
In the art of percussion drilling several attempts have been made to overcome the deficiencies of the use of end-to-end coupled single rod drill stems which transmit both percussion blows and bit rotation torque from the drill to the bit itself. U.S. Pat. No. 3,011,570 to E. H. Kurt, et al discloses a percussion drill stem assembly wherein an inner rod member transmits only percussion forces from the percussion drill motor to the drill bit or bit adapter and an outer tubular member forms a torque transmitting coupling between the drill motor rotation mechanism and the bit adapter. The Kurt patent discusses several advantages of the concentric drill stem assembly, namely the avoidance of the undue wear and early failure of the threaded couplings and the drill stem members themselves of the single rod type construction. The Kurt reference also describes the advantages of the essentially constant cross-section inner percussion member which minimizes the reflection of percussion forces transmitted from the drill motor to the bit. This reference does not, however, describe any means for isolating the inner rod member from the outer tubular torque transmission member in order to minimize the noise generated by the percussion transmitting member during drilling operations.
U.S. Pat. Nos. 3,981,368 and 4,094,364 to Hans Per Olof Lundstrom, et al also describe a concentric drill rod assembly for percussion drills wherein the inner percussion transmitting rod member is not of constant diameter or constant cross-section, but is provided with radial protrusions for locating or positioning the inner rod member with respect to the outer tubular member. Although the `364 patent to Lundstrom, et al describes a damping layer applied to the outer tubular member and the provision of elastomeric inserts supported by the protrusions on the inner rod member, the system of the Lundstrom, et al patents suffers from the disadvantage of inner rod members which are not of constant diameter or constant cross-section and therefore tend to be inefficient in that they reflect at least part of the compressional force pulses being transmitted by the inner rod member to the bit.
The further development of percussion drill rod assemblies and systems has sought not only to improve the operating life of these mechanisms by providing the concentric inner and outer percussion blow transmitting and torque transmitting members and the advantages that these members provide, but also to provide assemblies which minimize noise transmission from the drill rod assemblies to the operating environment of the drill, and to also improve the construction of these assemblies to minimize the need for special adapters or assemblies at the drill motor and at the percussion bit ends of the drill rod assemblies, respectively. Moreover, there has been a desire to utilize the advantages of the concentric drill rod assemblies in so-called one piece drill stems which are utilized with small handheld feed leg, stoper and similar types of percussion drills. It is to this end that the present invention has been developed with a view to providing improved percussion drill rod assemblies of the concentric inner and outer percussion blow and torque transmitting type as will be further appreciated by those skilled in the art from the further description herein.